1. Field of the Invention
The present invention relates to a technology for improving a dynamic balance of a so-called S-P-C type planetary gear mechanism.
2. Description of Related Art
A planetary gear mechanism is known as one of gear devices, in which an axis of an input shaft is made coincident with one of an output shaft. The planetary gear mechanism is, in general, made of three elements, i.e., a sun gear, a planetary gear and a carrier. The sun gear includes at least either one of an external gear and an internal gear (a ring gear). The planetary gear mechanism is classified into various types depending on the combination of above three elements.
FIG. 7 shows one type of the planetary gear mechanisms, which is called as an S-P-C type planetary gear mechanism. Here, reference numeral S designates a shaft of a sun gear, P designates a shaft of a planetary gear, and C designates a shaft of a carrier, wherein these shafts are arranged as a basic structure. In an example shown in FIG. 7, the shaft C of the carrier serves as an input shaft, and the shaft P of the planetary gear serves as an output shaft. The planetary gear is eccentrically, rotatably supported by the shaft C of the carrier. And, the planetary gear performs rotating and revolving motions while being engaged with the fixed sun gear (the ring gear). The shaft P of the planetary gear is provided with a universal joint J or a linking element equivalent thereto so as to abstract only the revolving motion as an output.
The S-P-C-type planetary gear mechanism gives a relatively large speed-reduction ratio (30:1) using only two gears (the ring gear and the planetary gear) in a state in which the input shaft is arranged coaxially to the output shaft, and therefore becomes highly useful in various fields. In addition, a so-called harmonic drive is also a gear device included in the S-P-C type planetary gear mechanism in the classification.
The S-P-C type planetary gear mechanism is essential for suppressing the vibrations associated with an eccentric motion of the planetary gear. FIG. 8 shows a conventional S-P-C type gear device with a construction aiming at suppressing the generation of the vibrations.
A gear device 1 is constructed as that a fixed ring gear 4 and planetary gears 5 and 5xe2x80x2 performing the rotating and revolving motions are disposed between an input shaft 2 and an output shaft 3, to thereby perform a speed-reduction of one stage. The input shaft 2 is provided with eccentric portions 2a and 2axe2x80x2 whereby the planetary gears 5 and 5xe2x80x2 are supported with an axis in an eccentric state. As shown in FIG. 8(a), if the eccentric portion 2a is made eccentric from the axis of the input shaft 2 by a distance a, the eccentric portion 2axe2x80x2 is made eccentric therefrom by the distance a correspondingly. And, the vibrations caused when the planetary gears 5 and 5xe2x80x2 perform the rotating and revolving motions while being engaged with the ring gear 4 are counteracted with each other.
As a linking element corresponding to the universal joint J shown in FIG. 7, a coupling plate 7 is disposed between a flange 6 fixed to the output shaft 3 and the planetary gear 5, and key slots 7a are provided on a surface and a reverse of the coupling plate 7 so as to intersect perpendicularly. A key 6a provided on the flange 6 and a key 5a provided on the planetary gear 5 are slidably engaged respectively with the key slots 7a on the surface and the reverse so as to constitute a universal joint. Although not illustrated in the drawing, a linking element for transmitting the rotating motion of the planetary gear 5xe2x80x2 to the output shaft 3 can be constructed by the same arrangement.
In the gear device 1 shown in FIG. 8, when the planetary gear 5 performs the rotating and revolving motions while being engaged with the ring gear 4, the planetary gear 5 and the planetary gear 5xe2x80x2 mutually function as a balancer, thereby counteracting their vibrations to each other.
However, the planetary gear 5 and the planetary gear 5xe2x80x2 are disposed to be offset from each other in the axial direction (i.e., the direction of axes of the input and output shafts 2 and 3). Consequently, the gear device shown in FIG. 8 secures a static balance among the rotating elements, but does not secure a dynamic balance in the axial direction among the rotating elements. Therefore, in a case the gear device 1 shown in FIG. 8 is required to be driven at a high speed, or under a use condition in which a precise operation is required, there may arise an adverse effect due to the vibrations caused by the dynamic imbalance in the axial direction among the rotating elements.
The present invention was made in order to solve the above-mentioned problem, and an object thereof is to improve an axial dynamic balance in a so-called S-P-C type planetary gear mechanism, thereby providing a gear device that can suppress the generation of the vibrations under various use conditions. Another object is to simplify the structural complication of the gear device and to provide the precise gear device at low cost.
In order to solve the above-mentioned problem, a gear device according to a first aspect of the present invention is a gear device which performs a speed-reduction of one stage using two gears disposed between an input shaft and an output shaft coaxially arranged, wherein a dynamic balance in an axial direction is secured for every individual rotating element.
In the present invention, the dynamic balance in the axial direction is secured for every individual rotating element, whereby the dynamic balance in the axial direction is secured for the entire gear device.
In the gear device according to a second aspect of the present invention, the rotating elements include a disk that has a circular recess arranged to be eccentric with respect to the input shaft and that is fixed to the input shaft, a ring gear that is rotatably supported by the circular recess and whose rotating motion is restricted so that the ring gear only makes an eccentric motion within the circular recess when the disk is rotated, and a planetary gear that is fixed coaxially to the output shaft and engaged with the ring gear, and a weight portion is formed in the disk to recover the axial dynamic balance which is lost by arranging the circular recess and the ring gear.
In this arrangement, the axial dynamic balance of the disk which is lost by arranging the circular recess and the ring gear is recovered using the weight portion. Moreover, since the planetary gear is fixed coaxially to the output shaft, it does not make the eccentric motion, thereby secures the dynamic balance thereof. Accordingly, the dynamic balance in the axial direction is secured for every individual rotating element.
In the gear device according to a third aspect of the present invention, the weight portion includes a weight-reduced portion formed in a portion of the disk where the recess is not provided, the weight-reduced portion having the same depth as that of the recess. According to the present invention, since the weight portion is formed by the disk per se, it is unnecessary to newly add a member onto the disk for attaining the axial dynamic balance of the disk. By setting the depth of the weight-reduced portion equal to the one of the recess, the axial dynamic balance of the disk is secured, which may otherwise be lost by arranging the circular recess in the disk.
In the gear device according to a fourth aspect of the present invention, the weight-reduced portions have a constitution of a plurality of circular holes having the same diameter. Because the weight-reduced portions are a plurality of circular holes having the same diameter, the number of processing steps for forming the weight- reduced portion is reduced.
In the gear device according to a fifth aspect of the present invention, the weight-reduced portions are constituted as that a plurality of circular holes have diameters gradually varied. By providing as that the weight-reduced portions have a plurality of circular holes with diameters gradually varied, more ideal balance can be attained.
In the gear device according to a sixth aspect of the present invention, the disk is constructed as being dividable in the axial direction as a divided member, thereby offering more capacity to select material for respective portions of the disk. Further, one of the divided members is processed to have the circular recess and the weight-reduced portion, and thereafter integrated with the other divided member, so that the thickness of the circular recess and the one of the weight-reduced portion are easily matched with each other.
In the gear device according to a seventh aspect of the present invention, the rotating element includes a planetary gear that is supported by an axis with an eccentric portion provided on the input shaft, and that is engaged with a fixed ring gear to perform rotating and revolving motions, and wherein the planetary gear is provided with a cavity portion within which a weight potion is provided to recover a balance of the planetary gear.
In this arrangement, by providing the cavity portion in the planetary gear, and locating the weight portion within the cavity portion, the balance of the planetary gear that is supported by the axis with the eccentric portion provided on the input shaft to make the eccentric motion is recovered within the planetary gear. That is, by securing the axial dynamic balance of the planetary gear per se, the axial dynamic balance of the entire gear device can be attained.
In the gear device according to an eighth aspect of the present invention, the weight portion is fixed to the eccentric portion of the input shaft such that the eccentric motion of the center of gravity of the weight portion is opposite to the eccentric motion of the center of gravity of the planetary gear. According to this arrangement, the center of gravity of the planetary gear and the one of the weight portion are constantly positioned to achieve the counterbalancing, and therefore it is possible to secure the axial dynamic balance of the planetary gear that is a rotating element of the gear device.